1. Field of the Invention
The present invention relates to subterranean cementing operations, and more particularly, to fluid loss control additives for cement compositions, and methods of using cement compositions comprising such fluid loss control additives in subterranean formations.
2. Description of the Prior Art
Hydraulic cement compositions are commonly utilized in subterranean operations, particularly subterranean well completion and remedial operations. For example, hydraulic cement compositions are used in primary cementing operations whereby pipe strings such as casings and liners are cemented in well bores. In performing primary cementing, hydraulic cement compositions are pumped into the annular space between the walls of a well bore and the exterior surface of the pipe string disposed therein. The cement composition is permitted to set in the annular space, thereby forming an annular sheath of hardened substantially impermeable cement therein that substantially supports and positions the pipe string in the well bore and bonds the exterior surfaces of the pipe string to the walls of the well bore. Hydraulic cement compositions also are used in remedial cementing operations such as plugging highly permeable zones or fractures in well bores, plugging cracks and holes in pipe strings, and the like.
In order for such well cementing operations to be successful, the cement compositions utilized may include a fluid loss control additive to reduce the loss of fluid, e.g., water, from the cement compositions when they contact permeable subterranean formations and zones. Excessive fluid loss, inter alia, causes a cement composition to be prematurely dehydrated, which may limit the amount of cement composition that can be pumped, decrease the compressive strength of the cement composition and negatively impact bond strength between the set cement composition and a subterranean zone, the walls of pipe and/or the walls of the well bore.
Contemporary synthetic fluid loss control additives are large, water-soluble polymers. An example of such synthetic fluid loss control additive is a fluid loss control additive consisting of copolymers of acrylamide (xe2x80x9cAAxe2x80x9d) and 2-acrylamido, 2-methyl propane sulfonic acid (xe2x80x9cAMPS(copyright)xe2x80x9d). However, certain AA/AMPS(copyright) copolymers are useful only in a limited number of operations, specifically those where the bottom hole circulating temperature (xe2x80x9cBHCTxe2x80x9d) ranges from about 90xc2x0 F. to about 125xc2x0 F. The BHCT ranges encountered in subterranean operations often involve temperatures outside such a range. Also, certain of these copolymers have a salt tolerance of only up to about 10%, making certain of them unsuitable for applications involving cement compositions comprising salts.
The temperature limitations of certain AA/AMPS(copyright) copolymers are believed to be the result of hydrolysis of the amide groups. The carboxylate groups formed by such hydrolysis convert the copolymers to materials which retard the set time of the cement and reduce the compressive strength of the set cement. Further, in the lower portion of the above-mentioned temperature range (between about 90xc2x0 F. to about 100xc2x0 F.), certain AA/AMPS(copyright) copolymers are even less effective as a fluid loss control additive, requiring inclusion of larger amounts of such additive than at higher temperatures. The addition of such copolymers directly affects the rheology of the resultant cement composition, as copolymers of acrylamide and AMPS(copyright) exhibit high viscosity and poor mixability, thus the inclusion of a sufficiently large amount of fluid loss control additive to create a cement composition having an acceptable fluid loss often leads to viscosity and pumpability problems.
Additionally, synthetic polymers may not comply with environmental regulations in certain regions of the world. For example, the use of polyamide polymers in the North Sea is problematic. One possible cause of this difficulty is the high molecular weight of such synthetic polymers.
The present invention provides cement compositions which demonstrate improved fluid loss characteristics, and methods for cementing in a subterranean formation using such cement compositions.
One method of the present invention comprises the steps of providing a cement composition comprising a hydraulic cement, water, and a fluid loss control additive comprising at least two polymers connected by a pH-sensitive crosslink; placing the cement composition into the subterranean formation; and permitting the cement composition to set therein.
One embodiment of the cement compositions of the present invention comprises a hydraulic cement, water, and a fluid loss control additive comprising at least two polymers connected by a pH-sensitive crosslink. Optionally, other additives suitable for inclusion in cement compositions may be added.
The objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of the preferred embodiments, which follows.
The present invention provides cement compositions that have improved fluid loss characteristics and methods of using such cement compositions in subterranean formations. While the compositions and methods of the present invention are useful in a variety of subterranean applications, they arc particularly useful for subterranean well completion and remedial operations, such as primary cementing, e.g., cementing casings and liners in well bores, including those in production wells, which include multi-lateral subterranean wells.
The cement compositions of the present invention generally comprise a hydraulic cement, water sufficient to form a pumpable slurry, and a fluid loss control additive of the present invention. The cement compositions of the present invention may range in density from about 4 lb/gallon to about 23 lb/gallon. In certain preferred embodiments, the density of the cement compositions may range from about 12 lb/gallon to about 17 lb/gallon. In certain other embodiments, the cement compositions can be low-density cement compositions, e.g., foamed cement compositions or cement compositions comprising other means to reduce their densities, e.g., microspheres.
Any cements suitable for use in subterranean applications are suitable for use in the present invention. In certain preferred embodiments, the improved cement compositions of the present invention comprise a hydraulic cement. A variety of hydraulic cements are suitable for use including those comprised of calcium, aluminum, silicon, oxygen, and/or sulfur, which set and harden by reaction with water. Such hydraulic cements include, but are not limited to, Portland cements, pozzolanic cements, gypsum cements, high alumina content cements, silica cements, and high alkalinity cements. In certain preferred embodiments, the hydraulic cement is a Portland cement.
The cement compositions of the present invention further comprise water, which can be from any source provided that it does not contain an excess of compounds that adversely affect other compounds in the cement compositions. For example, a cement composition of the present invention can comprise fresh water, salt water (e.g., water containing one or more salts dissolved therein), brine (e.g., saturated salt water), or seawater. The water may be present in an amount sufficient to form a pumpable slurry. More particularly, the water is present in the cement compositions of the present invention in an amount in the range of from about 16% to about 220% by weight of cement (xe2x80x9cbwocxe2x80x9d) therein. In certain preferred embodiments, the water is present in the cement compositions in the range of from about 30% to about 70% bwoc therein.
The cement compositions of the present invention also comprise a fluid loss control additive of the present invention, present in the cement composition in an amount sufficient to provide a desired level of fluid loss control. More particularly, the fluid loss control additive may be present in the cement compositions of the present invention in an amount in the range of from about 0.1% to about 5.0% by weight of the water in the cement composition. In certain preferred embodiments, the fluid loss control additive is present in the cement composition in an amount in the range of from about 0.3% to about 1.4% by weight of the water in the cement composition.
Generally, the fluid loss control additives of the present invention comprise two or more polymers connected by a pH-sensitive crosslink, e.g., a borate ester of polyvinyl alcohol. The pH-sensitive crosslink between the polymers is achieved through the use of a polyvalent cation. Any polyvalent cation capable of connecting two or more polymer strands through a pH-sensitive crosslink may be suitable for use with the fluid loss control additives of the present invention. One of ordinary skill in the art with the benefit of this disclosure will recognize the appropriate polyvalent cation for use in a particular application. In certain preferred embodiments, the polyvalent cation comprises a Group IIIA element such as boron or aluminum, or a Group IVB element such as titanium or zirconium. As used,herein, the terms xe2x80x9cGroup IIIA elementxe2x80x9d and xe2x80x9cGroup IVB elementxe2x80x9d will be understood to mean those elements depicted as belonging to either Group IIIA or Group IVB, respectively, as shown on the periodic table of the elements found in the endpapers of John McMurry, Organic Chemistry (2d. ed. 1988). In certain preferred embodiments where the polyvalent cation comprises boron, any source of borate ion may be used with the fluid loss control additives of the present invention, including, inter alia, borax, sodium borate, or boric acid. An example of a suitable source of borate ion is reagent grade boric acid, commercially available from Sigma Aldrich, Inc., at various locations.
The pH-sensitive nature of the abovementioned crosslink, inter alia, may improve the degradability of the fluid loss control additives of the present invention. The pH-sensitive nature of the crosslink causes the crosslinked polymers to fall apart in a solution of water having a pH within a particular range, depending on the polyvalent cation used to make the crosslink. In certain preferred embodiments wherein the polyvalent cation comprises boron, the crosslinked polymers fall apart in a solution of water having a pH below about 9.2. A typical cement composition will have a pH ranging from about 9.2 to about 13. Accordingly, the pH-sensitive crosslink present in certain preferred embodiments of the fluid loss control additives of the present invention is substantially stable when placed in a typical cement composition. However, when the pH-sensitive crosslink present in such preferred embodiments is placed into a source of free water, e.g., seawater, the crosslink between a higher molecular weight polymer and a lower molecular weight polymer is broken, thus releasing into the seawater a lower molecular weight polymer that is more likely to biodegrade.
Generally, the polymers used in the fluid loss control additives of the present invention have a minimum molecular weight of at least about 1,000. In certain preferred embodiments, the polymers comprise multiple polymers differing widely in molecular weight, e.g., the molecular weight of a first polymer may differ from the molecular weight of a second polymer by at least 100%. In one preferred embodiment, one polymer has a molecular weight of at least 80,000, while another polymer has a molecular weight of less than about 8,000. In this preferred embodiment, the polymer having the greater molecular weight is present in the fluid loss control additive in an amount ranging from about 100% to about 500% of the presence of the polymer having the lesser molecular weight. In one preferred embodiment, the fluid loss control additive comprises a borate ester comprising a 1:1 mixture of polyvinyl alcohols of widely different molecular weights, e.g., a polyvinyl alcohol having a molecular weight of about 140,000 and a polyvinyl alcohol having a molecular weight of about 5,000.
The polymers used in the fluid loss control additives of the present invention may comprise the same chemical compound, or different chemical compounds. For example, generally speaking, the polymers may comprise polyalcohols such as 1,2 alcohols and 1,3 alcohols. Suitable 1,2 alcohols include, inter alia, polysaccharides, such as guar gum. Suitable 1,3 alcohols include, inter alia, polyvinyl alcohols. The polymers may also, comprise, inter alia, alpha hydroxy acids and 1,2 amines. In certain preferred embodiments, the polymers used in the fluid loss control additives of the present invention comprise polyvinyl alcohols. An example of a suitable polyvinyl alcohol is a polyvinyl alcohol having a molecular weight of about 5,000, commercially available from Cross World Sale Corporation, of Mohegan Lake, N.Y., under the tradename xe2x80x9cERKOL 03/140.xe2x80x9d Another example of a suitable polyvinyl alcohol is a polyvinyl alcohol having a molecular weight of about 140,000, commercially available from Cross World Sale Corporation, of Mohegan Lake, N.Y., under the tradename xe2x80x9cERKOL 40/140.xe2x80x9d
Generally, the fluid loss control additive of the present invention is made by dissolving one or more polymers in water; adding a polyvalent cation to the solution; and adjusting the pH as necessary to crosslink the polymers until the resulting solution achieves a desired molecular weight. One of ordinary skill in the art with the benefit of this disclosure will be able to identify suitable methods of measuring molecular weight of the solution, and will be able to recognize when a sufficient degree of crosslinking has been achieved. One such suitable method of detection is multi-angle light scattering HPLC.
The fluid loss control additives of the present invention may be added to the cement compositions of the present invention in a variety of ways. The dry cement, water and fluid loss control additive may be mixed in any order and given sufficient time to let the fluid loss control additive hydrate. The dry materials will typically swell when contacted with water; thus, an appropriate waiting period for hydration is typically a period of about 10 minutes after the end of visible swelling.
Optionally, the cement compositions of the present invention may be low-density cement compositions. For example, the cement compositions of the present invention may comprise foamed cement compositions. Where the cement composition is foamed, foaming agents and/or foam stabilizing agents, or mixtures thereof, may be included in the cement composition in order, inter alia, to facilitate the foaming and/or enhance the cement composition""s stability. The foaming agent and/or foam stabilizing agent is generally present in the cement composition in an amount sufficient to provide a stable foamed cement composition. One of ordinary skill in the art with the benefit of this disclosure will recognize the appropriate type of foaming agent and/or foam stabilizing agent for use in a particular application, along with the amount in which such agent or agents should be incorporated.
Where the cement compositions of the present invention comprise foamed cement compositions, an expanding additive may be included in the cement composition. The expanding additive may be any component that enables a gas to become incorporated into the cement composition. Further, the addition of the expanding additive to the cement composition can be accomplished by any suitable method. In one preferred embodiment, the cement is foamed by direct injection of the expanding additive into the composition. For instance, where the cement composition is foamed by the direct injection of gas into the composition, the gas utilized can be air or any suitable inert gas, such as nitrogen, or even a mixture of such gases. In certain preferred embodiments, nitrogen is used. In other preferred embodiments, the cement is foamed by gas generated by a reaction between the cement slurry and an expanding additive present in the cement in particulate form. For example, the composition may be foamed by hydrogen gas generated in situ as the product of a reaction between the high pH slurry and fine aluminum powder present in the cement. Where an expanding additive in particulate form is used, aluminum powder, gypsum blends, and deadburned magnesium oxide are preferred. Preferred expanding additives comprising aluminum powder are commercially available under the tradenames xe2x80x9cGAS-CHEK(copyright)xe2x80x9d and xe2x80x9cSUPER CBLxe2x80x9d from Halliburton Energy Services, Inc., of Duncan, Okla.; a preferred expanding additive comprising a blend containing gypsum is commercially available under the tradename xe2x80x9cMICROBONDxe2x80x9d from Halliburton Energy Services, Inc., of Duncan, Okla.; and preferred expanding additives comprising deadburned magnesium oxide are commercially available under the tradenames xe2x80x9cMICROBOND Mxe2x80x9d and xe2x80x9cMICROBOND HTxe2x80x9d from Halliburton Energy Services, Inc., of Duncan, Okla. Such preferred expanding additives are described in U.S. Pat. Nos. 4,304,298; 4,340,427; 4,367,093; 4,450,010 and 4,565,578, which are assigned to the assignee of the present application and are incorporated herein by reference. The amount of expanding additive present in the cement composition is that amount which is sufficient to incorporate a desired amount of a gas into the cement composition so that the cement composition has a density in the desired range. A density in the range of from about 4 to about 20 pounds per gallon is suitable. One of ordinary skill in the art with the benefit of this disclosure will recognize the proper amount of an expanding additive to use in order to provide a foamed cement composition having a desired density.
Another example of a low-density cement composition of the present invention is one that comprises microspheres. Any microspheres that are compatible with a subterranean cement composition, i.e., that are chemically stable over time upon incorporation into the cement, may be used. An example of a suitable microsphere is commercially available from Halliburton Energy Services, Inc., of Houston, Tex., under the tradename xe2x80x9cSPHERELITE.xe2x80x9d Where included, the microspheres are present in the cement composition in an amount sufficient to provide a cement composition having a density in a desired range. More particularly, the microspheres may be present in the cement composition in an amount in the range of from about 10% to about 150% by weight of the cement. The microspheres may be added to the cement composition by any suitable method including by dry blending with the cement before the addition of a fluid such as water, by mixing with the fluid to be added to the cement, or by mixing with the cement slurry consecutively with or after the addition of the fluid. The microspheres may be pre-suspended in water and injected into the cement mix fluid or into the cement slurry as an aqueous slurry.
As will be recognized by those skilled in the art, the cement compositions of this invention also can include additional suitable additives, including, inter alia, accelerants, set retarders, defoamers, weighting materials, dispersants, vitrified shale, fly ash, and/or formation conditioning agents. One of ordinary skill in the art with the benefit of this disclosure will recognize the proper additives to be used in a particular application, along with the proper amounts. Although all of these additives are suitable, it has been found that additives comprising strong sulfonates may not be the most suitable for use in conjunction with the cement compositions of the present invention, as they tend to cause the cement composition to prematurely gel.
An example of a cement composition of the present invention comprises Class H Portland cement, 0.5% xe2x80x9cCFR-3xe2x80x9d dispersant by weight of the cement, 0.5% xe2x80x9cD-AIR 3000xe2x80x9d defoamer by weight of the cement, 1.0% of a fluid loss control additive of the present invention comprising a borate ester of polyvinyl alcohols by weight of the cement, and 47.5% water by weight of the cement. CFR-3 is a dispersant commercially available from Halliburton Energy Services, Inc., of Duncan, Okla. D-AIR 3000 is a defoamer commercially available from Halliburton Energy Services, Inc., of Duncan, Okla.
A method of the present invention comprises providing a cement composition that comprises a hydraulic cement, water sufficient to form a pumpable slurry, and a fluid loss control additive of the present invention; placing this cement composition in a subterranean formation; and permitting the cement composition to set therein.
To facilitate a better understanding of the present invention, the following examples of some of the preferred embodiments are given. In no way should such examples be read to limit the scope of the invention.